Patent GB-1492582-A (GB)
Title: Camera tube circuit
IPC: H04N5/228
Filed: 1974-09-04
Abstract: 1492582 Cathode-ray tube circuits PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 4 Sept 1974 [7 Sept 1973] 38657/74 Heading H4T The invention is concerned with a circuit arrangement for use with a television pick-up tube provided with an anti-comet tail (ACT) electron gun which is so controlled that, during the line fly back period, which precedes the actual beam blanking interval, the beam scans the target with an increased beam diameter (defocused spot) and an increased intensity the cathode potential also being increased so that charges due to locally excessive exposures of the target are completely neutralized. As shown, the camera tube 1 includes a cathode c, control electrode g 1 , first anode g 2 , lens electrode g 3 , second anode g 4  (connected to g 2 ), collector anode g 5 , g 6  and target tg. The anode g 2 , g 4  is connected to a source of potential V s1  and also to the input 6 of the circuit arrangement, according to the invention, which has an output 7 connected to the control electrode g 1 . The signal U g1  is supplied to the control electrode by generator 8 which is connected to a pulse generator 9. The latter is controlled by a field scanning signal R and a line scanning signal L. As shown in Fig. 2, the line blanking time T B  is divided into line-flyback time T BS  and beam-blanking time T BB , the signal B being used for beam blanking. The pulse generator 9 also supplies signals A, C, D, E, F. The ACT gun generates an electron beam e 2  during the flyback time T BS  by virtue of the fact that the voltage U gl  is raised to the slightly negative potential -Vb g  l  whereby the current intensity of beam e 2  is hundreds of times larger than that of the scanning beam e 1 . Also, during part T BS1  of the time T BS  a low positive potential Vb g3  is applied to the lens g 3  to generate a node in the beam e 2 . In order to prevent erasure of information during line flyback, the cathode c must be at a positive potential Vbc which determines the potential of the electron beam landing spot, whereby the potential image is neutralized (erased) to this potential. For generating the signal U gl , the generator 8 is supplied with B, C and a control signal Vb g1  from the circuit 11, 12, 13, 14. The input to this circuit at terminal 6 (the signal H) is applied to a clamping circuit 11 controlled by signal A. The output of circuit 11 is sampled by circuits 12, 13 during respective periods T BS1  and T BS2  under control of the signals E, F respectively and the resultant signals M, N (shown as D.C. levels in Fig. 2) are compared in a comparator 14 to which a reference bias - V g1  is also applied. The output of comparator 14 is a signal P = N-M-V g1 , having a low positive value +p volts. The voltage p determines the value of a resistance PR in the circuit 8, Fig. 3, such that the lower the voltage p the less negative the voltage U g1  during the period T BS . The erasure beam e 2  is effective only during period T BS1 . During the period T BS2  the beam e 1  is generated with a cathode potential + Vbc and a control electrode potential - Vb g1  whereby a large current flows through resistance 4 to the grid g 2 , g 4  giving rise to the signal K. The difference between the voltages on g 2 , g 4  in the periods T BS1  and T BS2  is indicated by the voltage k which is a measure of the maximum current intensity of the beam e 2  available for erasure. An optimum value of the voltage k may be obtained by adjustment of the reference bias V g1 . Fig. 2b shows the result of a change in the cathode bias Vbc which, manually or otherwise, is increased to the value Vbc<SP>1</SP>. The signals H and K are reduced in amplitude and the resultant change in signal P causes a small increase in the resistance PR which continues until the period shown in Fig. 2c is reached. The output of the circuit 8 is a voltage - Vb g1 <SP>1</SP> slightly less negative than the voltage - Vb g1  resulting in an increase in beam intensity during the period T BS . This causes an increase in amplitude of signals H and K which, at the period denoted by Fig. 2d, become equal to the values for the period denoted by Fig. 2a A decrease in the value of U c  causes converse transition phenomena and, again, a restoration of k to optimum value.